Technologies and systems are known in the art that are applied to machines adapted for packaging different products with plastic-type heat-shrinking films. These machines, both of the manual and of the automatic types, use plastic heat-shrinking films with different thicknesses. A source of heat produced by electric resistances heats an air circuit taking the machine to the suitable temperature for shrinking the heat-shrinking film. This hot air flows onto the heat-shrinking film into which the product to be packaged is wrapped and makes the film shrink, performing the adherent packaging of the product inside the plastic film.
Machines are further known that use, for shrinking the films, sources of heat realised through infrared-rays-emitting diffusers.
Other systems, according to the type of product to be packaged, use, as heat source, hot water, or high-temperature steam.
The above machines, adapted for packaging various products with heat-shrinking films, have, in the majority of cases, an arrangement comprising a basement that supports coils of heat-shrinking film (single-bend type or not) and that contains a film-welding frame, and a conveyor or roller belt for passing the product inside an hot-air circulating oven. Obviously, the products, before passing into the shrinkage area (with hot air or the like), are wrapped, in the packaging area, by the heat-shrinking film, and this latter one is welded in its open edges by means of the welding frame with hot welding blades. Afterwards, the product, wrapped into the heat-shrinking film, passes through the oven, where heat is emitted, and then shrinks around the product itself. These machines can be of the manual, semi-automatic and completely automatic types.
Another type of known packaging is the so-called “bundle” type. The product is wrapped by a heat-shrinking film in a single sheet and shaped as a bundle; then, the film is welded and cut, and afterwards the film-wrapped product passes into the heating oven, where the film, stably adhering to the product, is shrinked.
The process that allows sending hot air onto the heat-shrinking film and other systems (for example infrared rays) can create some difficulties in the positive outcome of a constant film heat-shrinkage. In fact, into hot-air circulating ovens, hot air cannot be constantly and smoothly sent onto the whole film, and therefore its shrinkage does not perfectly occur, and in some cases there remain unshrinked film edges on the product, impairing the packaging appearance.
Moreover, the ovens must be perfectly insulated, and, in spite of modern insulation technologies, hot air goes out of the product insertion areas and out of the product output areas. This inconvenience can create pollution from heat in the environment where these machines operate.
The above problems occur for infrared heating systems, that, in addition to the above-described inconveniences, have the problem of propagating also on the product to be packaged, since the plastic film, being of a transparent type, lets these infrared rays pass with consequent energy waste. GB-A-1 015 716 discloses a method of packaging a column of disc-shaped articles that uses, among others, a pulsating magnetic field to heat objects so that a shrinkable film can be shrinked over the heated objects.
EP-A-1 013 551 a method and apparatus to package objects with a heat-shrinkable sheet which is an example of the above-mentioned prior art.